For nearly four years, detectives using 38 laptop computers tracked thousands of telephone calls between reputed mob figures, the union officials they are accused of bribing and contractors charged with helping them siphon millions of dollars from construction projects around New York City.

The investigators said they also collected tens of thousands of financial records, conducted hundreds of surveillances, downloaded the hard drives of 276 computers and subpoenaed so many documents that at one point they had to store them in a 40-foot tractor-trailer.

Last week, their efforts led to what they called New York City's most significant construction racketeering indictment focusing on the mob in a decade. The indictments, obtained by Robert M. Morgenthau, the Manhattan district attorney, were the result of an unusual amount of resourcefulness, payroll hours and technology-based work.

That work, coupled with long hours poring over financial records, helped build the case, which charges mob figures with bribing union officials to allow contractors to pay workers less than the union wage and forgo expensive benefits.

Extensive and detailed as the investigators' work has been, the legal process is still at an early stage. Most of the 38 defendants pleaded not guilty to charges including enterprise corruption and restraint of trade, beginning what will probably be a long series of hearings before any of the men go to trial.

"I think a lot of people don't really understand how much work goes into an extensive investigation like this," said Daniel J. Castleman, the chief of Mr. Morgenthau's Investigations Division, who oversaw much of the case.

At times, there were simultaneous court- authorized taps on 18 different phones.

A detective monitoring each tapped line sat in front of a laptop as it displayed the numbers of incoming and outgoing calls. Sitting side by side, the detectives listened to the tapped lines over headphones as cassette decks recorded the conversations. At the same time, teams of investigators were ready to move quickly to observe meetings arranged in the same conversations, often in language they had to decode.

The indictment, which charged union officials, contractors and men the authorities identified as mob figures, was the result of the work of more than a dozen detectives, as well as several prosecutors in Mr. Morgenthau's office.

Most of those who worked on the investigation acknowledge that the driving force was a soft-spoken detective who, like many organized-crime figures, traces his roots in part to the neighborhood around Pleasant Avenue in East Harlem.

The investigator, Robert Callus, had spent years kicking down doors as a narcotics detective before he was transferred into the organized-crime unit in 1992 and began schooling himself on the mob and the construction industry.

A muscular and tattooed 40-year-old whose formal education ended when he graduated from John Bowne High School in Flushing, Queens, he said that soon after he was assigned to investigate mobsters, he read labor racketeering's great works, including cases from the 1980's targeting the concrete and window-replacement industries.

Detective Callus, who was the primary investigator on the case, known as Operation Textbook, also learned at the shoulder of Anthony Carro, a forensic financial analyst in the district attorney's office whom Detective Callus called "a wizard."

Mr. Carro showed him how to use the payroll records of construction companies and union financial records to gather evidence about how some racketeering schemes siphoned off millions.

He spent most of his days in a dreary rented office in Queens where the detectives monitored the wiretaps. There, tiers of cassette decks tied to the Toshiba laptops ringed a room about 25 by 35 feet. Above each computer, taped to the dingy white walls, were instructions about each tapped phone.

During the summer of 1997, Detective Callus spent much of his time on the roof of a six-story building near the site of a high- rise apartment building under construction.

He said he and his colleagues watched and photographed organized-crime figures meeting with union officials at the construction site, and were amused to see a 79-year- old man, whom the authorities described as a Lucchese family soldier, working as a flagman.

Detective Callus said that after a lot of work the rooftop observations yielded information that helped obtain wiretaps on the phones of union officials and organized- crime figures. While he often saw those targets using cell phones at the job site, conducting what he believed was mob business, the phones were not listed in the users' names.

To obtain a judge's permission to wiretap those phones, Detective Callus reviewed calls made to and from the suspects' homes around dinner time. "Married men, when they're out past dinner time, will call home," he said. "I thought, `When do I call my wife?'"

Once he determined the numbers of the cell phones used in those dinner-time calls, and from that the names of the phones' owners, he could work to establish a connection between the cell phone owner and the union official or reputed mob figure who was using that phone.

And while much of the work was dull, slow and plodding, there were moments of excitement, he said. One arose when investigators had to make sure that a reputed mob figure sat at the right table in a fast-food restaurant  the one they had bugged.

Steven L. Crea, one of the defendants in the case, whom prosecutors have identified as the acting boss of the Lucchese crime family, was regularly meeting union officials and mobsters at a Roy Rogers restaurant on Bruckner Boulevard in the Bronx, and the investigators wanted to place a bug there to pick up his conversations. But a judge was reluctant to authorize the listening device because he believed that the investigators could not guarantee that it would be placed where Mr. Crea would sit.

But Detective Callus was able to convince the judge that he could direct his target to the bugged table with a series of careful chesslike moves, occupying and vacating certain tables and making others unappealing by spilling soda on them.

As a result, Mr. Crea ended up at the table over and over again and had about a dozen conversations that were secretly recorded and may be used as evidence.

Sometimes, the music played in the restaurant was so loud that it threatened to obscure the whispered conversations, so an undercover investigator complained angrily to an employee, and the volume was turned down.

The Roy Rogers conversations were the first the police obtained recording unguarded participants.

Detective Callus is no stranger to Mr. Crea, since he participated in investigations involving Mr. Crea, at least peripherally, since 1992. But the two men apparently reached an unusual peace earlier this year.

When the detective and other investigators came to Mr. Crea's office at 645 Bronx River Road in the Bronx to execute a search warrant, he and Mr. Crea talked briefly, according to court papers filed last week.

Mr. Crea seemed concerned that the police were focusing on his son, who is in business with him, and told the detective so. "Steve, I assure you I am not targeting your son," the detective said. "If there is nothing there, nothing will happen. Did I ever come after you when there was nothing there?"

Mr. Crea, whose son was not charged in the case, responded, "No, and I always respected you for that."